References

Edmunds Jr, GF. 1995 Habitat differences between northern and southern populations of mayflies of the western United States. Pages 171-176 in Corkum LD; Ciborowski JJH. Current Directions in Research on Ephemeroptera. Canadian Scholars' Press, Inc. Toronto.

Hagen,HA 1861 Synopsis of the Neuroptera of North America with a list of South American species. Smithsonian Miscellaneous Collections 4, 1-344.
Hagen described Caenis amica for the first time in this publication. Hagen's Glossary (pdf)

McCafferty,WP; Durfee,RS and Kondratieff,BC 1993 Colorado mayflies (Ephemeroptera): an annotated inventory. Southwestern Naturalist 38 3, 252-274. PDFQuote from page 268 "Colorado is one of the few areas in North America from which this ubiquitous species had been previously unreported (Provonsha, 1990)." The authors also mention at the end of their discussion of Faunisitics that C. amica is one of "those mayfly species that are truly widespread in North America"

Webb,JM; Jacobus,LM; Funk,DH; Zhou,X; Kondratieff,BC; Geraci,CJ; DeWalt,RE Baird,DJ Richard,B Philips,I and Hebert,PDN 2012 A DNA barcode library for North American Ephemeroptera: Progress and prospects. PloS One 7(5): e38063 HTMLUnder the section titles "Barcode Distances between Species", the authors state: "Among Caenis amica Hagen, C. punctata McDunnough, and C. youngi Roemhild, the interspecific distances were as low as 0.3% (maximum intraspecific distances ranged from 3.7–21.9% and none of the species were monophyletic), possibly an artifact of incomplete taxonomic knowledge or historical introgression. For nearly all other species, the minimum interspecific distances were much greater (mean: 12.5%)." In other words these three species may actually be one, but further collecting and DNA analysis is nessecary to answer this question.